Regardless of which class is tanking, it must be remembered that the primary job of the tank is to hold aggro. Much as healers must avoid the temptation to nuke, the tank must avoid the temptation to try and duel one mob at a time. The tank absolutely must realize that they are there to keep the mobs off of all other party members if possible. If you see an add go after your healer, it's your job to pull it off. Far too often Tanks will just stand there, slugging it out with one mob, while the rest of the party dies. A tank in a good group should be able to keep aggro on every mob attacking except under special circumstances where someone else is chosen to have aggro. As long as you have damage dealers to dish out damage and a healer to keep you alive, the one and only thing that you need to worry about is making sure that every mob in the fight is hitting you, not someone else.

You need to do your best to mitigate damage so that the healer isn't streaming their mana into you. Increase your dodge, parry, block and melee defense ratings the best you can. Every tank class has a "tank mode" (Defensive Stance, Bear Form, Frost Presence, Righteous Fury), and should always be in it. The only exception consists of dropping out for a fraction of a second to use an ability like Innervate or Berserker Rage.

Remember to change your targets. Once you have generated sufficient aggro, you should move onto the next in the attacking group (not an idle mob out of combat) so that when the rest of your party is done killing the primary target, you have already instilled some aggro in the next one.

Keep an eye on every mob and make sure they are all attacking you or possibly the secondary tank(s), and not someone else. Watch for remarks such as "add" or "on me" in the party/raid channel. "Add" means that new unexpected mobs have just joined the combat. Not managed properly, adds often lead to wipes. "On me" (or "aggro") means that one of the mobs broke aggro and is attacking someone who's not supposed to be attacked, usually a healer or a caster.

Keep your camera zoomed out as far as possible, to get a broader view of the battlefield and see if anyone else is drawing aggro.

What the other group members need to know to assist the Tank

Do not pull adds. The tank has a hard enough time without you pulling extra mobs on to him. Assist the tank, and kill what he's fighting. If you're a healer, know the tank's health bar better than your own.

Spellcasting mobs are difficult for a tank keep aggro on, since they stay at range, out of reach of most of the tank's threat building abilities. If you have a ranged silence or interrupt ability (Silence, Counterspell, Strangulate, Silencing Shot, etc), the tank will probably be grateful if you use it on the caster. A silenced mob will run toward its target. Death knights can also Death Grip casters, but be careful, as this will force it to attack you for a few seconds.

Avoid unplanned AoE damage, as the tank will usually be anticipating high damage on one target at a time. In encounters where AoE is ideal, plan the fight with the rest of the team so that everyone has the same priorities.

If you're under attack, do not run, unless it is to go straight to the tank and wait. Running does nothing but make it harder for the tank to target the mobs attacking you. Stand still and only use threat-reducing abilities.

If the tank doesn't notice you're being attacked, say something. The phrase "on me" or "aggro" is commonly used in this situation.

Allow time for the tank to build aggro before attacking. Generally, healers should try to avoid healing the tank for the first few seconds of the fight, and other classes should avoid doing massive amounts of damage or threat at the outset.

If you are a caster with a lot of spell damage or spell crit gear and/or higher level than your main tank, damage from your critical hits will occasionally outweigh threat caused by the tank, and the mob will go after you. In raids, this is usually instant death. Several classes have a threat-wiping ability, and be prepared to use it preemptively. If you die your DPS is 0.

Warlocks and hunters, turn off auto-Torment/auto-Growl on your pet, since this draws mobs away from the tank and prevents him or her from being able to build aggro. Remember that while your pet could absorb some damage, the tank is the player chosen for that role for the party.

Attack only the mob selected by the leader, or by assisting a the party's selected main assist; the tank will not anticipate high damage on other targets. Here is the simplest macro possible for assisting another character: